Regulation of submergence-induced enhanced shoot elongation in Oryza sativa L

Abstract

Rice (Oryza sativa L.) is the only cereal that can be cultivated in the frequently flooded river deltas of South-East and South Asia. The survival strategies used by rice have been studied quite extensively and the role of several phytohormones in the elongation response has been established. Deep-water rice cultivars can diminish flooding stress by rapid elongation of their submerged tissues to keep up with the rising waters. Other rice cultivars may react by mechanisms of submergence tolerance. Aerenchyma and aerenchymatous adventitious roots are formed that facilitate oxygen diffusion to prevent anaerobic conditions in the submerged tissues. This paper discusses the molecular aspects of the mechanism that leads to shoot elongation (leaves of seedlings and internodes), the regulation of which involves metabolism of, and interactions between, ethylene, gibberellins and abscisic acid. Finally, the importance of new techniques in future research is assessed. Current molecular technology can reveal subtle differences in gene activity between tolerant and non-tolerant cultivars, and identify genes that are involved in the regulation of submergence avoidance and tolerance.

Fig. 1. The ethylene biosynthetic pathway. SAM, S‐adenosyl‐l‐methionine; MTA, 5′‐methylthioadenosine; ACC, 1‐aminocyclopropane‐1‐carboxylic acid; MACC, 1‐(malonyl)aminocyclopropane‐1‐carboxylic acid; GACC, 1‐(glutamyl)aminocyclopropane‐1‐carboxylic acid.

Fig. 2. Histochemical analysis of β‐glucuronidase (GUS) activity in transgenic rice carrying an OS‐ACS5–GUS fusion. A, In air‐grown rice seedlings (left) OS‐ACS5–GUS was expressed at a low level in the leaf sheath (LS) and in response to complete submergence the expression in vascular bundles of young leaf sheaths and laminae was strongly induced (right). B, Vascular bundle in mature leaf sheath (8‐week‐old plant). Phloem parenchyma cells (PP) surrounding the sieve tubes (ST) and companion cells (CC) in the phloem show GUS‐expression. Also the thin‐walled xylem parenchyma cells (XP) that envelope the protoxylem lacuna (PL) and the thick‐walled XP cells that are in contact with the metaxylem vessels (MX) show GUS‐activity. CC, Companion cell; G, ground tissue; L(1 or 2), leaf (first or second); LS, leaf sheath; M, mestome sheath; MX, metaxylem vessel; P, phloem; PL, protoxylem lacuna; PP, phloem parenchyma cell; ST, sieve tube; X, xylem; XP, xylem parenchyma cell.

Fig. 3. A, Schematic representation of the morphology of a 9‐d‐old air‐grown seedling, with two leaves (left), and a 1‐week‐older plant with the third leaf appearing (right). C, Coleoptile; L(1, 2 or 3), leaf (first, second or third). B, Schematic representation of a longitudinal section of the upper part of a stem containing the apical meristem and the youngest internodes from growing rice plants.